netfilter: nf_tables: use u32 for chain use counter

Since 4fefee5 ("netfilter: nf_tables: allow to delete several objects
from a batch"), every new rule bumps the chain use counter. However,
this is limited to 16 bits, which means that it will overrun after
2^16 rules.

Use a u32 chain counter and check for overflows (just like we do for
table objects).

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>